1. Field of the Invention
The present invention relates to a display apparatus, or more particularly to a busy lamp field for displaying the state of a subscriber terminal accommodated in an exchange, and for connecting a call from a station terminal to the subscriber terminal.
2. Description of the Related Art
In recently developed electronic exchange systems (e.g., a private branch exchange or PBX), a busy lamp field (hereinafter called BLF) is widely used to display the subscriber state (e.g., an idle state and called and recalled states as explained below) of a plurality of subscriber terminals or telephones (hereinafter called subscribers or SUB) accommodated in the exchange. The BLF is also used by an operator to connect a call originated from a station terminal to a subscriber.
Accordingly, a BLF that can effectively display the subscriber state and can connect a plurality of calls to the subscribers in an arbitrary order, is in great demand.
FIG. 1 is a block diagram of an electronic exchange system as described above. A PBX 2 accommodates a plurality of subscribers 1-i (where i=1, 2, . . . , mn) and is equipped with an attendant console 4 (hereinafter called ATT) and a busy lamp field 3 (BLF) which is usually provided near the ATT 4. The ATT 4 is used by an operator to connect a called subscriber 1-i or a subscriber in the recalled state 1-i (explained below) to an incoming call from a calling station terminal 9 via an incoming trunk 8.
For a PBX 2 accommodating a small number (n) of subscribers 1-i, a BLF 3 with only a subscriber keyboard 31 is used. The subscriber keyboard 31 has n subscriber keys 311, and each key 311 has a built-in lamp or light-emitting-diode (LED), with the key top thereof being covered with a translucent pad. Each lamp in a key 311 stays lit, flickers and stays off to indicate subscriber states of "busy", "ATT calling" and "idle", respectively. When a subscriber 1-i is in a state where it is being called by a station terminal 9 but is not connected to the station terminal 9 (hereinafter referred to as a called state), the lamp in the subscriber key 311 corresponding to the subscriber 1-i lights up to inform the operator of the called state. When the operator presses the lit subscriber key 311, the subscriber 1-i is connected to the ATT 4.
For a PBX 2 accommodating a greater number (n.times.m) of subscribers 1-i, a BLF with both a subscriber keyboard 31 and a plane-switch keyboard 32 is typically used. The subscriber keyboard 31 has n subscriber keys 311 arranged in a matrix and each key 311 has the above-described construction. Hereinafter, a group of n subscribers 1-i whose subscriber states are displayed on the subscriber keyboard 31 at one time and the subscriber states of the group displayed are both referred to as a plane. The plane-switch keyboard 32 has m plane-switch keys 321 arranged in a line, and each key 321 has the same construction as the keys 311. The plane-switch keys 321 are used to switch between the m different planes which can be displayed on the subscriber keyboard 31, allowing the BLF 3 to display a total of n.times.m subscriber states for n.times.m subscribers 1-i.
The following is a description of an example of a BLF 3 with both a subscriber keyboard 31 and a plane-switch keyboard 32. If a call to a subscriber 1-i is suspended and the called state continues for a predetermined time (hereinafter, this subscriber state is called a "recalled state"), the subscriber 1-i is put in the recalled state. If a plurality of subscribers 1-i are to be put in the recalled state, they are put in the recalled state in a system-defined order (e.g., on a first-come, first-served basis). When a subscriber 1-i is in the recalled state, the plane-switch key 321 that corresponds to the plane to which the subscriber 1-i belongs, is lit. Pressing the lit plane-switch key 321 causes the subscriber key 311 corresponding to the subscriber 1-i to light up, thereby informing the operator of the recalled state of the subscriber 1-i. Then, pressing the lit subscriber key 311 connects the suspended call to the subscriber 1-i.
In addition, when a subscriber 1-i is put in the recalled state, a ringer on the ATT 4 rings to call the operator's attention to the recalled state. When the operator presses a "recall" key (not shown) on the ATT 4, the suspended call is connected to the subscriber 1-i.
However, there is a problem with the above-described system in that when a plurality of subscribers 1-i are in the recalled state, the operator can recognize only one recalled state subscriber 1-i. Therefore, there is no room for the operator to recognize and select a subscriber 1-i having a higher priority for connection to a suspended call, from among the remaining recalled state subscribers 1-i.